1. Field of the Invention
The present invention relates to a thermal head, and more particularly to a thermal head for a thermal transfer printer or a thermal printer with the improvement of printing quality of a printed product obtained thereby.
The thermal head for utilizing the thermal transfer printer or the thermal printer of the present invention is single thermal head having a partial projected glaze portion mounted on a head substrate member or a thermal head having a plurality of partial projected glaze portions mounted on a head substrate member, such as a dual type thermal head, a zigzag or staggered type thermal head etc..
2. Description of the Prior Art
The conventional thermal head for the thermal transfer printer is disclosed in, for example, Japanese Patent Laid-Open Publication No. 59093/1983, in which the thermal head has partial projected glaze portions having a semi-column cross-sectional shape form so as to form a plurality of heating element rows thereon.
FIG. 6 shows a sectional view of a conventional thermal head having two heating resistor element rows 132 and 133, in which the thermal head is pressed with a platen 118. The thermal head comprises a head substrate member 131, semi-column cross-sectionally shaped two partial projected glaze portions 134 and 135 mounted on the head substrate member 131, and two heating resistor element rows 132 and 133 formed on the upper sides of the partial projected glaze portions 134 and 135, respectively.
For the simplification of explanation, protective films of the heating resistor element rows 132 and 133, the ink ribbon in which the protective films of the heating resistor element rows 132 and 133 lie between the heating resistor element rows 132 and 133 and the platen 118, and the recording paper are omitted from in FIG. 6.
As shown FIG. 6, there is no problem when a center between the partial projected glaze portions 134 and 135 and a center between the heating resistor element rows 132 and 133 have no offset value or has a small offset value therebetween.
However, because of the manufacturing dispersion of the thermal head, as shown in FIG. 7, the center between the partial projected glaze portions 134 and 135 and the center between the heating resistor element rows 132 and 133 has a comparative large offset value therebetween. The interval (L.sub.0) shows distance between the adjacent partial projected glaze portions 134 and 135 and is about 2-4 mm in length. The width (W.sub.0) shows width of the partial projected glaze portion 134 or 135 and is about 0.8-1 mm in length. The height (H.sub.0) shows height of the partial projected portion 134 or 135 and is about 30-60 .mu.m in height.
When the heating resistor element rows 132 and 133 has the comparative large offset value respectively toward the printing direction, non-contact area portion or a low contact pressure area portion with the recording paper is created between the heating resistor element row 132 or 133 and the platen 118.
In FIG. 7, the recording paper is omitted, however the above inconvenience or tendency is developed even though under the consideration of the affect of the recording paper. As a result, the transfer or recording dot shape becomes worse, and there is an inconvenience that the unbalance phenomenon occurs in the result of printing product because of the difference of the contact conditions of the heating resistor element rows 134 and 135 with the recording paper.
In the conventional thermal head, the surface of the heating resistor element row is projected roundly all over in a line with the semi-column cross-sectionally shaped partial projected glaze portion and the contact area pressure portion of the heating resistor element row with the recording paper is designed uniformly at the printing time.
Therefore, the transfer or recording dot shape may reproduce almost the heating resistor element shape. However, generally the transfer or recording dot shape is lengthened toward the movement direction or the electrode direction of the thermal head in comparison with the heating resistor element shape when the thermal head runs continuously. The more the transfer or recording dot shape lengthens toward the movement direction or the electrode direction of the thermal head, the more the recording speed of the thermal head is increased, as a result the the printing product quality by the thermal head becomes poor.
FIG. 14 shows a sectional view of the thermal head under the printing time, in which the conventional partial projected glaze portion structure is used. The ink ribbon is omitted therefrom. A resistor film 155, a common electrode 153, an individual control electrode 164, and a protective film 156 are coated on a partial projected glaze portion 151 in order. The partial projected glaze portion 151 is disposed on a head substrate member 150.
As shown this figure, a heating resistor element 152 contacts all over with recording a paper 114. When the thermal head does not move, only one dot is transferred on the recording paper 114, the recording dot having a similar dimension that of the heating resistor element 152 would be obtained. However, generally the thermal head prints under the condition that the thermal head moves.
By the electric current time of the heating resistor element 152 and the heat transmission time of the protective film 156 and the ink ribbon, the recording or transfer dot 157 is lengthened toward the thermal head movement direction in proportion to the movement amount of the heating resistor element 15. (see FIG. 15). The length d.sub.1 shows the starting condition of the heating resistor element 152 and the length d.sub.2 shows the condition of the heating resistor element 152 when one dot is continued.
In the conventional thermal transfer printer, as stated above, the thermal transfer printer having the high speed and the high printing product quality is not taken into consideration.
For the sake of the miniaturization and the low price of the integral circuit (IC) apparatus for driving the thermal head and also the restraint of electric current loss of the heating resistor element driving electric power etc., it is necessary to carry out under high voltage and low electric current driving, in which the resistance value of the heating resistor element is increased. The resistance value of the heating resistor element depends on the kind of material, dimension or film thickness of the heating resistor element itself.
The correction of the lengthening of the recording dot through the dimension of the heating resistor element can be done by shortening of the distance between the common electrode end portion and the individual control electrode end portion of the heating resistor element, because of both of the common electrode end portion of the heating resistor element and the individual control electrode end portion of the heating resistor element are drawn toward the lateral direction or the printing direction. However, when the lateral or side dimension of the heating resistor element shortens, the resistance value of the heating resistor element becomes low.
As a result, the electric current amount under the electric current increases and the electric current capacity of the IC apparatus for driving the thermal head becomes large, an exothermic or heating resistor amount of the thermal head concentrates in the heating resistor element having a short lateral dimension or a short printing direction dimension, and the life of the heating resistor element is reduced.
By making the film thickness of the heating resistor element of the thermal head thinner, the resistance value of the heating resistor element may be increased. However, the exothermic or heating resistor amount of the thermal head concentrates at the thinner film thickness portion, as a result the life of the heating resistor element of the thermal head is reduced.